Automated analyzers are widely used, which irradiate a reaction solution into which a sample and a reagent are mixed with light from a light source, calculate absorbance from a change in the amount of transmitted light of a specific wavelength, and quantify the concentration of a substance to be measured in a blood sample according to the Lambert-Beer law (see PTL 1, for example). These automated analyzers are provided with multiple cells holding the reaction solution on the circumference of a cell disc that repeats rotation and stop, obtain time-series data of the amount of light transmitted through the reaction solution in the cell as reaction process data for about 10 minutes at intervals of about 15 seconds in a transmitted light measuring unit disposed at a predetermined position during rotation of the cell disc, calculate absorbance from a change in the amount of the light, and qualify the concentration of a substance to be measured.
An automated analyzer mainly measures two types of reactions: a color reaction between a substrate and an enzyme and an immune reaction between an antigen and an antibody. Analysis using the former reaction is referred to as a biochemical analysis, and its test items are lactate dehydrogenase (LDH), alkaline phosphatase (ALP), aspartate aminotransferase (AST), and the like. Analysis using the latter reaction is referred to as immunoassay, and its test items are C-reactive protein (CRP), immunoglobulin (IgG), rheumatoid factor (RF), and the like. Test items that are required for quantification in the low range of blood concentration are present in some substances measured by the latter. For such test items, latex immunoassay is performed that uses latex particles on the surface of which antibodies are sensitized (bound) as a sensitizer (see PTL 2, for example).
In the latex immunoassay, the antibody on the surface of latex particles contained in a reagent recognizes and binds the antigen as a substance to be measured contained in a sample, and as a result the latex particles aggregate through the antigen and the aggregate of latex particles are produced. A conventional automated analyzer irradiates a reaction solution into which aggregates are dispersed with light, and measures the amount of transmitted light that passes through without being scattered by the aggregates of the latex particles. The higher the concentration of the antigen, the size of the aggregate after a certain period of time increases, so that more light is scattered and the amount of the transmitted light is reduced. Thus, the concentration of the antigen can be quantified from the amount of light measured as reaction process data.
Higher sensitivity of the latex immunoassay is desired in recent years and hence the measurement of scattered light rather than transmitted light has been attempted. For example, a system is disclosed that separates transmitted light and scattered light using a diaphragm and measures the absorbance and the scattered light at the same time (see PTL 3). The diameter and the like of reagent particles suitable for the measurement of scattered light are also disclosed (see PTL 4).